Axial turbine engines generally include fan section, compressor, combustor and turbine sections positioned along a centerline referred to as the engines “axis of rotation”. The fan, compressor, and combustor sections add work to air (also referred to as “core gas”) flowing through the engine. The turbine extracts work from the core gas flow to drive the fan and compressor sections—typically via concentric drive shafts. The fan, compressor, and turbine sections each include a series of stator and rotor assemblies. The stator assemblies, which do not rotate (but may have variable pitch vanes), enable proper aerodynamics of the engine compressor and turbine by guiding core gas flow into or out of the rotor assemblies.
Current and projected future trends enabling improved fuel efficiency will tend to increase the number of compression stages and/or mechanical speeds to enable increased stage loading and/or as a means of increasing cycle overall pressure ratio. These changes may exacerbate existing challenges in shaft dynamics and rotor critical speeds as the compressor, combustor, and turbine sections increase in length and/or reduce in diameter. The net result may increase the length and decrease the diameter of the shaft; thus, creating an adverse effect on shaft critical speed and torque carrying capabilities. There is therefore a need for an engine architecture that improves fuel efficiency without creating adverse design and dynamic effects on the engine shaft.